boost/pfr/detail/fake_object.hpp
// Copyright (c) 2023 Bela Schaum, X-Ryl669, Denis Mikhailov. // // Distributed under the Boost Software License, Version 1.0. (See accompanying // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) // Initial implementation by Bela Schaum, https://github.com/schaumb // The way to make it union and UB free by X-Ryl669, https://github.com/X-Ryl669 // #ifndef BOOST_PFR_DETAIL_FAKE_OBJECT_HPP #define BOOST_PFR_DETAIL_FAKE_OBJECT_HPP #pragma once #include <boost/pfr/detail/config.hpp> #ifdef __clang__ # pragma clang diagnostic push # pragma clang diagnostic ignored "-Wundefined-internal" # pragma clang diagnostic ignored "-Wundefined-var-template" #elif defined(__GNUC__) # pragma GCC diagnostic push # pragma GCC diagnostic ignored "-Wundefined-var-template" #endif namespace boost { namespace pfr { namespace detail { // This class has external linkage while T has not sure. template <class T> struct wrapper { const T value; }; // This variable servers as a link-time assert. // If linker requires it, then `fake_object()` is used at runtime. template <class T> extern const wrapper<T> do_not_use_PFR_with_local_types; // For returning non default constructible types, it's exclusively used in member name retrieval. // // Neither std::declval nor boost::pfr::detail::unsafe_declval are usable there. // This takes advantage of C++20 features, while boost::pfr::detail::unsafe_declval works // with the former standards. template <class T> constexpr const T& fake_object() noexcept { return do_not_use_PFR_with_local_types<T>.value; } }}} // namespace boost::pfr::detail #ifdef __clang__ # pragma clang diagnostic push #elif defined(__GNUC__) # pragma GCC diagnostic pop #endif #endif // BOOST_PFR_DETAIL_FAKE_OBJECT_HPP